The present invention relates to a method of measuring the adhesion of a coating to a substrate, and in particular of a metallic coating deposited on a metallic substrate.
The adhesion of a coating to a substrate can be assessed using test pieces made up of disks of the same material as the substrate with the coating on one of their sides. These disks are bonded onto counter-test pieces that are fixed to the jaws of a tensile testing machine. The measurement of the adhesion of the coating is then supplied by a mechanical measurement of the force needed to tear the coating from the substrate.
However, considerable discrepancies in the tensile strength measured on identical test pieces are observed, which shows that the method used introduces uncertainties which are great enough for it to be difficult to attach any value to these tests. It is probable that the bonding is responsible for defects of uniformity of transmission of the tensile force through the assembly but this cannot be remedied even by taking the greatest care in creating the bond.
Furthermore, with certain tungsten carbide type coatings which offer an adhesion greater than that of the bonding agent used, these tests measure the tensile strength of the bond and not that of the coating.